This invention relates to screen baskets for screening apparatus as encountered in the pulp and paper industry. The screen baskets separate rejectable material from acceptable material when both are in a liquid suspension.
In the papermaking process, a screening apparatus is typically used to separate wood knots and other foreign material from a fluid pulp. A typical screening apparatus includes a housing within which a screen basket is mounted around a concentrically positioned rotor assembly. The screen basket is usually fabricated from a relatively thin metal plate material although bar or wire materials are also often used and when mounted in a screening apparatus, provides a barrier between a screening chamber and an accept chamber. Fluid pulp is introduced to the screening chamber by way of a feed inlet which is upstream of the screening chamber. The fluid pulp is introduced to either the inner or the outer portion of the screen basket depending on the type of screening apparatus used. Material which does not pass through the screen basket flows to an end of the screening chamber away from the feed inlet and is removed through a reject outlet. One type of screen basket has circular shaped openings sized to reject unwanted solids and may have support rings located along the length of the basket to provide additional mechanical support. Another type of screen basket has slots having lengths much greater than their widths for separating other types of materials and may have support rings located along the length of the screen basket to provide additional mechanical support.
The rotor assembly generally includes hydrofoils or a contoured drum mounted on a rotating shaft in close proximity to the screen basket to sweep past the openings of the screen basket. The hydrofoils or contoured drum may be positioned to sweep over the inner or the outer surface of the screen basket. The rotating hydrofoils or contoured drum generate hydrodynamic pulses in the radial direction with enough force and frequency to continuously remove any fiber plugs that occur in the screen basket openings. The localized flows caused by the hydrodynamic pulses are generally in a direction opposite to the flow of the fluid pulp provided to the screen basket under pressure.
End rings are generally attached to end portions of the screen basket for mounting the screen basket into the screening apparatus. The end rings are bolted to a stationary member to prevent the screen basket from rotating in response to the torsional forces generated by the rotating hydrofoils or drum. Such a mounting arrangement generally places the screen basket in an axially compressive loading condition.